Electronic phase separation in (La1/3Sm2/3)2/3A1/3MnO3 (A = Ca, Sr and
Ba) compounds
Saket Asthana, D. Bahadur, A. K. Nigam, and S. K. Malik
Citation: J. Appl. Phys. 97, 10H711 (2005); doi: 10.1063/1.1853277
View online: http://dx.doi.org/10.1063/1.1853277
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JOURNAL OF APPLIED PHYSICS 97, 10H711 共2005兲
Electronic phase separation in „La1/3Sm2/3…2/3A1/3MnO3
„A = Ca, Sr and Ba… compounds
Saket Asthana and D. Bahadura兲
Department of Metallurgical Engineering and Materials Science, Indian Institute of Technology,
IIT Bombay, Powai, Mumbai 400076, India
A. K. Nigam and S. K. Malik
Tata Institute of Fundamental Research, Colaba, Mumbai 400005, India
共Presented on 8 November 2004; published online 11 May 2005兲
Electrical and magnetotransport properties of 共La1/3Sm2/3兲2/3A1/3MnO3 共A = Ca, Sr, and Ba兲
compounds, synthesized by the citrate gel route, have been investigated. These compounds
crystallize in an orthorhombic structure 共space group Pnma兲. Semiconducting-like behavior is
observed only in Ca-substituted sample while Ba and Sr-substituted samples show metal-insulator
type transition. The high and weakly temperature dependent magnetoresistance at low temperature,
thermal irreversibility in field-cooled and zero-field-cooled magnetization, and nonsaturating
magnetization behavior are expected in systems with electronic phase separation. The cationic size
disorder by varying alkaline earth ions causes the electronic phase separation. © 2005 American
Institute of Physics. 关DOI: 10.1063/1.1853277兴
INTRODUCTION
Rare earth manganites of the type ABO3 exhibit many
interesting properties such as charge-ordering, colossal magnetoresistance, electronic phase separation, etc.1,2 The phase
separation in manganites has attracted the attention of theoreticians as well as experimentalists. The A-site cation disorder is responsible for the lattice distortion which leads to the
localization of eg electrons, which in turn, leads to electronic
phase separation in these materials.2 Mixing cations of different sizes and charges at the A site is the most straightforward experimental method for systematically tuning the
properties of these materials.3
In this article we present the results of varying alkaline
earth ions 共keeping the rare earth ratio fixed兲 on the magnetic
and transport properties of some manganites. The series,
共La1/3Sm2/3兲2/3A1/3MnO3 共A = Ca, Sr, and Ba兲, was selected
for the present study. The A-site ionic radius and its associated cation disorder are the varying parameters in these compounds.
EXPERIMENTAL DETAILS
Polycrystalline samples of 共La1/3Sm2/3兲2/3A1/3MnO3 共A
= Ca, Sr, and Ba兲 were synthesized by the chemical citrategel route using high purity La2O3, Sm2O3, CaCO3, BaCO3,
SrCO3, and Mn-acetate. The as prepared powders were calcined at 1000 ° C in air for 2 h. The powders were palletized
in the form of rectangular bars and sintered at 1200 ° C in air.
X-ray diffraction patterns of the samples were recorded using
Cu K␣ radiation 共PANalytical, PW 3040/60 Philips兲. Resistivity measurements at different applied magnetic fields were
carried out between 5 and 320 K using the standard fourprobe dc method 共PPMS兲 共Quantum Design兲. Magnetic meaa兲
Author to whom correspondence should be addressed; electronic mail:
[email protected]
0021-8979/2005/97共10兲/10H711/3/$22.50
surements were made using a vibrating sample magnetometer 共Oxford兲 at different fields and in the temperature range
5 – 300 K.
RESULTS AND DISCUSSION
All the three samples are found to be single phase crystallizing in the orthorhombic perovskite structure 共space
group Pnma, No. 62兲. The Ca-substituted sample shows
higher orthorhombicity than the other two samples which in
turn, is due to smaller ionic size of Ca ion 共1.34 Å兲 in comparison to that of Sr 共1.44 Å兲 and Ba 共1.61 Å兲 ions.4 The cell
parameters were refined by the Rietveld method. The refined
lattice parameters are given in Table I.
In 共La1/3Sm2/3兲2/3A1/3MnO3 共A = Ca, Sr, and Ba兲 series,
lattice distortion occurs in the three dimensional MnO6 octahedra on varying the alkaline earth ions. The A-site average
ionic radius, 具rA典, varies from 1.30 Å for Ca, 1.33 Å for Sr,
and 1.39 Å for Ba-substituted samples. The tolerance factor
共t兲 is the ratio of A-O and B-O bond distances in the ABO3
perovskite which is responsible for the lattice distortion. The
tolerance factor increases from Ca- to Ba-substituted
samples. The increase of the Mn–O–Mn bond angles enhances the hopping of eg electron between Mn3+ and Mn4+
sites via double exchange which favors the ferromagnetic
共FM兲 metallic phase. The ionic radius of O2− is 1.32 Å which
is comparable to Sr-substituted sample 共具rA典 = 1.33 Å兲 but
somewhat different from 具rA典 of Ba 共1.39 Å兲- and Ca
1.30 Å-substituted samples. This mismatch between the sizes
may cause lattice distortion and, hence, electronic inhomogeneity at microscopic scale in these samples.2 The distortion
of Mn–O–Mn bond angle caused by lattice distortion, leads
to the charge localization effects for Ba- and Ca-substituted
samples. Further, it may also cause antiferromagnetic 共AFM兲
interaction due to change in Mn–O–Mn bond angle and distance. Thus the development of AFM regions within the pre-
97, 10H711-1
© 2005 American Institute of Physics
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10H711-2
J. Appl. Phys. 97, 10H711 共2005兲
Asthana et al.
TABLE I. Refined lattice parameters, a,b,c using Rietveld method, tolerance factor t, TMI, TC and percentage of
MR in 8 T field for the series, 共La1/3Sm2/3兲2/3A1/3MnO3 共A = Ca, Sr, and Ba兲.
Systems
共La1/3Sm2/3兲2/3Ca1/3MnO3
共La1/3Sm2/3兲2/3Sr1/3MnO3
共La1/3Sm2/3兲2/3Ba1/3MnO3
a 共Å兲
b 共Å兲
c 共Å兲
t
TMI 共K兲
TC 共K兲
%MRmax
5.4084共3兲
5.4722共5兲
5.5056共1兲
7.6394共2兲
7.7063共7兲
7.7801共7兲
5.4528共6兲
5.4599共6兲
5.5022共8兲
0.9620
0.9742
0.9950
¯
196
56
⬃50
200
100
⬃95
⬃80
⬃99
dominantly FM regions causes electronically inhomogeneous
regions at the microscopic scale.5 The earlier-mentioned FM
and AFM interactions compete with each other and may be
responsible for frustration in the system.
The temperature dependence of electrical resistivity,
␳共T兲, for 共La1/3Sm2/3兲2/3A1/3MnO3 共A = Ca, Sr, and Ba兲
samples in zero and 8 T fields is shown in Fig. 1. In zero
field, semiconducting-like behavior 共d␳ / dT ⬍ 0兲 is observed
only in the Ca-substituted sample, down to 70 K below
which the resistance of the sample was beyond measurement
limit. Typical metal-insulator 共MI兲 type transition is observed
in Sr- and Ba-substituted samples at temperature 共TMI兲 of
196 and 56 K, respectively. In the Ba-substituted sample, the
semiconducting-like behavior reappears at some temperature
below TMI. In the 8 T field, ␳共T兲 of Ca-substituted sample,
becomes nearly temperature independent at low temperatures
which indicates the reappearance of FM ordering. High but
weakly temperature dependent magnetoresistance 共MR兲 behavior is observed at low temperatures in all the cases as
shown in inset of Fig. 1. Table I gives the values of TMI and
maximum MR 共in 8 T field兲 values for all the compounds.
The
resistivity
data
of
the
series
共La1/3Sm2/3兲2/3A1/3MnO3 共A = Ca, Sr, and Ba兲 are found to be
well described by the Mott–VRH model in which
ln ␳␣T−1/4.6 This is shown in Fig. 2. The resistivity in the
metallic region 共in Ba- and Sr-substituted samples兲 has been
fitted to a Zener-double exchange polynomial ␳ = ␳0 + ␳2T2
+ ␳nTn, where ␳0 is the temperature independent residual resistivity due to scattering by impurities, defects, grain boundaries, and domain walls, ␳2T2 is ascribed to the electronelectron and electron-phonon mechanisms,7 and ␳nTn
corresponds to the electron-magnon scattering. The value of
FIG. 1. Variation of normalized electrical resistivity with temperature for
the series 共La1/3Sm2/3兲2/3A1/3MnO3 共A = Ca, Sr, and Ba兲. Inset shows the MR
behavior with temperature.
n for the higher order term has been found to be 4.5 in our
systems. The fitted data in metallic region are shown in the
inset of Fig. 2.
The plots of magnetization as a function of temperature
for 共La1/3Sm2/3兲2/3A1/3MnO3 共A = Ca, Sr, and Ba兲 compounds
are shown in Fig. 3. The FM to paramagnetic transition temperature, TC in all cases is determined from the minimum in
dM / dT 共Table I兲. The TC is not the same as TMI especially in
Ba- and Ca-substituted samples. The discrepancy between TC
and TMI is indicative of destruction of long range FM ordering and the latter leads to charge localization at low
temperatures.8 Thermal irreversibility effect in magnetization, which is due to the competing AFM and FM interactions, is observed below TC in these compounds. There is
strong irreversibility behavior between the zero-field-cooled
共ZFC兲 magnetization and the field cooled 共FC兲 magnetization. The inset in Fig. 3 shows the inverse susceptibility versus temperature plots in 0.01 T for all the samples. It is
interesting to note that the susceptibility of Ba- and Casubstituted samples deviates from the Curie–Weiss 共C-W兲
law while that of Sr-substituted sample does not.
The deviation from C-W law is larger in the case of Ca
compound compared to that in Ba compound. This deviation
from C-W law indicates the presence of magnetic inhomogenities which is usually taken to be an evidence of electronic
phase separation.9
Figure 4 shows field dependence of magnetization for
共La1/3Sm2/3兲2/3A1/3MnO3 共A = Ca, Sr, and Ba兲 samples at 5 K.
The magnetization reaches near saturation value in Srsubstituted sample. However, the magnetization of Basubstituted sample approaches saturation rather slowly 共compared to the Sr-substituted sample兲 while that of Casubstituted sample is not saturated even in 5 T field. This
FIG. 2. Theoretical fitting of the experimental resistivity data of
共La1/3Sm2/3兲2/3A1/3MnO3 共A = Ca, Sr, and Ba兲 compounds.
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10H711-3
J. Appl. Phys. 97, 10H711 共2005兲
Asthana et al.
FIG. 3. Temperature dependence of magnetization in a field of 0.01 T in
ZFC and FC runs for 共La1/3Sm2/3兲2/3A1/3MnO3 共A = Ca, Sr, and Ba兲 compounds. Inset shows the plot of inverse of susceptibility vs temperature.
magnetization behavior of Ca- and Ba-substituted samples
support the presence of microscopic inhomogenities due to
distortion in MnO6 octahedra, though this effect is more
prominent in Ca containing compound. The maximum magnetization of Sr- and Ba-substituted compounds shows near
saturation with values of 3.45 and 3.42 ␮B per formula unit,
respectively, which are close to the theoretical value of
3.7 ␮B / f.u. The Ca-substituted sample exhibits a very small
magnetic moment of 0.08 ␮B / f.u.. giving a strong evidence
of electronic phase separation.10,11
CONCLUSIONS
We have investigated the structural and the magnetotransport properties of 共La1/3Sm2/3兲2/3A1/3MnO3 共A = Ca, Sr,
and Ba兲 compounds. The irreversibility in ZFC and FC magnetization behavior in all the samples and strong dependence
of resistivity on magnetic field at low temperatures 共below
100 K兲 are suggestive of electronic phase separation in the
Ba- and Ca-substituted samples. The electronic inhomogenities at microscopic scale are maximum for Ca containing
compound and least for Sr case. This is attributed to size
effects which cause distortion of MnO6 octahedra as
confirmed by susceptibility, temperature dependent MR, and
FIG. 4. Isothermal variation of magnetization with field for the series
共La1/3Sm2/3兲2/3A1/3MnO3 共A = Ca, Sr, and Ba兲 at 5 K.
M-H behavior. The two competing interactions, namely, the
lattice distortion that favors charge localization 共insulating
phase兲, and FM ordering which favors metallic phase, presumably lead to electronic phase separation.
ACKNOWLEDGMENT
Two of the authors S.A. and D.B. are thankful to the
Department of Science and Technology 共DST兲 India for support of the project.
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